OK ... Weight Sensing Prop. Valve on Rear of 86 ??

[wildman7798]

In the process of swapping my 86 4.10 open with the 94 3.55 Trac Loc. Got the old diff. out and getting ready to install the 94. So my 86 has the hillbilly hydraulic precursor to anti-lock brakes in the way of a rotary valve with some crazy tinker toy linkage tied to the 10.25 diff. cover. My first thought was it was aftermarket then doing searches I see it was actually Fords idea, apparently to help avoiding wearing out front pads and to avoid the dreaded skidding rear tires. Seems like I saw some posts here months ago that say leave it alone. I really can't see any need to have this and even if I leave it who is to say it works and how would I ever know it works down the road?? Looks like a problem waiting to happen to me. There must be many of you in IDI land that have dealt with this. My issue is I don't see a hole anywhere in the back crossmember (before the rear tank) where the hard line and the soft hose can connect and use a "C" clip. There is a small plate bracket the valve bolts to, I could probably remove and use the drill press and get the correct hole for the soft hose flange to fit. It will require some serious bending of that hardline to get all this to work. So tell me why I should keep this crap or drive it like every other truck built before 1986 and manage my own controls.

 

[gandalf]

That "thing" you speak of, mounted on the read axle, is an OEM piece of equipment. I believe it is called a proportioning valve. The theory was that it changed the amount of braking power assigned to the read brakes, depending on load. The heavier the load and therefore the lower the rear end was, the more braking power went to the rear brakes.

My '86 had one, but the brakes were so poor that I don't know whether it worked or not. Add to that the fact that I had 2500 pound constant duty overload springs, so the rear end never did squat and activate it anyway.

All that being said, my suggestion would be to leave it on there. In the event that you should ever have an accident, the other guy's insurance company can say, "Look!! He altered the braking system on his truck." Almost by definition then, the accident is your fault. We're talking liability here.

 

[wildman7798]

In theory it could only decrease the braking power to the rear shoes. It is the same M/C up front, same prop valve up front, same single line running from front to back. So it fits in between the left and right rear lines, same skinny little rear lines. It would seem to me that the only time it could (by design) do anything, is to flow less fluid when there is no weight on the truck, there by lessening the chance of skidding the back tires (even though the fronts do 60% of the braking in disc/drum system). By looking at it I see no possible way it can increase or enhance the breaking power when loaded beyond not having it in the system at all. It is not a fluid supercharger, it can only make it flow up to wide open (same as not being there). With all that said... I fully agree with Gandalf about liability... however I have spent 40 years cutting and hacking and building my rigs better, faster, stronger than those penny pinching, cookie cutter car factories could ever do.

I really believe this was a failed attempt at a pre ABS system. My 74 F250 460 Camper Special never needed it, My 78 F250 4x4 never needed it, and these trucks up into the 70's and late 80's all had essentially the same brakes. Same rotors, same M/C's same calipers, and either 2 1/2" or 3" rear shoes. It was what.... ? 1987 when they finally put ABS on the back and that really never worked well or at all on most of the trucks until the early 90's. Just sayin....

Here is the last point to consider.. does anyone actually believe they could stop their empty or loaded F250 IDI safer and in a shorter distance by having less fluid going to the back shoes?

 

[mohavewolfpup]

Here is the last point to consider.. does anyone actually believe they could stop their empty or loaded F250 IDI safer and in a shorter distance by having less fluid going to the back shoes?

Some hood rat cut my truck off once, slamming the brakes on did a fairly good job stopping it. Took years off already bad tires in the back and smoked the area some, but it stopped!

 

[gandalf]

There is the possibility, with those old proportioning valves, that we were getting only 75%, for example, of total possible fluid to the rear when not loaded. Then when we loaded the rear, squatted it a bit, we increased that to 100% of what little fluid flow was possible. Now that's a scarey thought.

I remember once testing the effectiveness of the brakes on my '86. We were building the addition on the house, and I was hauling away the last of the dirt excavated from the foundation. I had a bed full of dirt, mounded a bit, and was pulling a Bobcat. On San Tomas Expressway, for those of you familiar with the area, some woman just ahead of me was foolish enough to hit her brakes as soon as she saw a yellow light. Most people would have kept going through the yellow, but not her. I had to do a panic stop, foot on the brakes, shoulders on the seat back, butt in the air, trying to pull the steering wheel up by its roots. I stopped without hitting her, but only barely. I had to return home to change pants.